Separator for secondary battery

ABSTRACT

Provided herein is a separator used for an electrochemical device such as a lithium-ion battery. The separator disclosed herein comprises a porous base material, and a protective porous layer coated on one or both surfaces of the porous base material disclosed herein, wherein the protective porous layer comprises an organic binder and an inorganic filler, and wherein the inorganic filler comprises a whisker-type material selected from the group consisting of Al 2 O 3 , SiO 2 , TiO 2 , ZrO 2 , BaO x , ZnO, CaCO 3 , TiN, AlN, MTiO 3 , K 2 O.nTiO 2 , Na 2 O.mTiO 2 , and combinations thereof, wherein x is 1 or 2; M is Ba, Sr or Ca; n is 1, 2, 4, 6 or 8; and m is 3 or 6. Also provided herein is a lithium-ion battery including the separator disclosed herein. The separator disclosed herein is excellent in terms of safety, ion permeability, and cycle characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.15/603,518, filed on May 24, 2017, which claims the benefit of U.S.provisional application No. 62/341,079, filed on May 25, 2016, both ofwhich are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

This invention relates to a separator used for an electrochemical devicesuch as a lithium-ion battery and a lithium-ion battery including theseparator disclosed herein. More particularly, the present inventionrelates to a separator in which a protective porous layer made of amixture of an organic binder and inorganic filler is formed on one orboth surfaces of a porous base material, and an electrochemical devicecontaining the separator disclosed herein.

BACKGROUND OF THE INVENTION

Lithium-ion batteries (LIBs) have been widely utilized in variousapplications especially consumer electronics such as laptop computers,mobile phones, and digital cameras, etc. Recently lithium-ion batterieshave started to be used in automobiles due to their superior energy andpower density.

LIBs generally include an anode, a cathode, a separator, and anelectrolyte. The anode and cathode are separated from one another by aseparator in order to prevent short circuit while maintaining ionconductivity.

A separator is conventionally a thin, porous, electrically insulatingmaterial having high ion permeability, good mechanical strength andlong-term stability to the chemicals and solvents used in the system,for example electrolyte of the electrochemical cell.

Separator for use in high performance battery system must be safe sincevery large quantities of energy are stored in the fully charged state inthe battery. These energies must not be released in an uncontrolledmanner in the event of malfunctioning of the battery, such asovercharging or short-circuit, since this would lead to an explosion orignition of the battery.

Generally, a typical organic separator consists of a composite filmcomprising a polyolefin-based substrate and an inorganic coating layer.A major disadvantage of these polyolefin-based separators is their lowthermal stability limit. When the battery temperature exceeds 150° C. orlower, the organic separator rapidly shrinks. A cathode and an anodewill directly contact to each other, to cause enlargement ofshort-circuited area. Therefore, such a separator is prone to causebattery short circuit and is generally not safe.

As a result, it is necessary to provide a separator that does not causeheat shrinking at high temperature.

JP Pat. No. 5617609 has disclosed a separator having a shutdownfunction. The separator comprises an ultra-high molecular weightpolyethylene (UHMWPE), high-density polyethylene (HDPE) and an inorganicoxide, and has a shutdown temperature of approximately 134° C. which isthe melting point of the HDPE. However, because such separator is stilla polyolefin-base material, they have a disadvantage of obtaining asignificant improvement in safety including prevention of heat shrinkingat high temperature. Therefore, there is a great possibility ofshort-circuit between a cathode and an anode caused by shrinking ormelting of separators in the event of an accident.

In another aspect, a separator must be strong enough to withstand thetension of the winding operation during battery assembly. In order toprevent possibility of damage such as a tear of the separator during amanufacturing process, different attempts have been made to solve theproblem and improve the performance of the separator includingmechanical strength, abrasion resistance and flexibility, etc.

U.S. Patent Application No. 20140212727 A1 has described a separatorhaving a good mechanical stability for the ease of handling in batterymanufacture. The separator comprises fibers of an electricallynonconductive material, and a porous electrically nonconductive coatingcomprising oxide particles, an inorganic adhesive and polymer particles.The separator shows improved laminatability in contact with electrodes,improved flexibility, higher tolerance to buckling, and greaterpenetration strength. However, the inorganic adhesive has been preparedvia a particulate sol or a polymeric sol. This makes the manufacturingprocess more complex. In addition, only some physical properties, suchas MacMullin number and Gurley Number, of the separator have beenmeasured. However, the physical properties of the separator alone do notmean superior battery performance.

U.S. Patent Application No. 20150380705 A1 has described a polyolefinseparator which has high tensile strength and low melt shrinkage in themachine direction (MD) and the transverse direction (TD) throughregulation of a stretching process. This involves subjecting theseparator to TD relaxation to remove stress from the separator subjectedto the TD stretching. However, regulation of a stretching process is notsuitable for separator that operates at high speed on a roll-to-rollprocess. Furthermore, the separator made of polyolefin will easilyshrink even at a temperature of about 150° C. or lower.

In view of the above, there is always a need to develop separators whichhave distinctly improved mechanical and thermal stability and aresuitable for mass production to meet the performance requirement inlithium high power batteries.

SUMMARY OF THE INVENTION

The aforementioned needs are met by various aspects and embodimentsdisclosed herein. In one aspect, provided herein is a secondary-batteryseparator comprising a porous base material and a protective porouslayer coated on one or both surfaces of the porous base material,wherein the protective porous layer comprises an organic binder and aninorganic filler, and wherein the inorganic filler comprises awhisker-type material selected from the group consisting of Al₂O₃, SiO₂,TiO₂, ZrO₂, BaO_(x), ZnO, CaCO₃, TiN, AlN, MTiO₃, K₂O.nTiO₂, Na₂O.mTiO₂and combinations thereof, wherein x is 1 or 2; M is Ba, Sr or Ca; n is1, 2, 4, 6 or 8; and m is 3 or 6.

In some embodiments, the whisker-type material has a diameter from about0.05 μm to about 3 μm. In certain embodiments, the whisker-type materialhas a ratio of length to diameter from about 5 to about 300.

In certain embodiments, the whisker-type material is randomlydistributed throughout the protective porous layer. In some embodiments,the whisker-type material is unidirectionally oriented in the TDdirection. In other embodiments, the whisker-type material tends toalign along the TD direction. In further embodiments, at least 50% ofthe whisker-type material are lying within an angle of ±30° about anaxis in the TD direction.

In some embodiments, the inorganic filler further comprises aparticulate-type material selected from the group consisting of Al₂O₃,SiO₂, TiO₂, ZrO₂, BaO_(x), ZnO, CaCO₃, TiN, AlN, and combinationsthereof, wherein x is 1 or 2. In certain embodiments, theparticulate-type material has an average diameter from about 100 nm toabout 10 μm. In some embodiments, the particulate-type material has anaverage diameter from about 1 μm to about 10 μm.

In certain embodiments, the protective porous layer is a single-layeredstructure. In some embodiments, the particulate-type material and thewhisker-type material are randomly distributed throughout the protectiveporous layer.

In some embodiments, the protective porous layer is a two-layeredstructure comprising a first layer and a second layer, wherein the firstlayer is adjacent to the porous base material, and the second layer ison and in contact with the first layer. In certain embodiments, theparticulate-type material resides in the first layer and thewhisker-type material resides in the second layer. In other embodiments,a substantial portion of the particulate-type material resides in thefirst layer and a substantial portion of the whisker-type materialresides in the second layer.

In some embodiments, the average thickness ratio of the second layer tothe first layer is from about 1:3 to about 3:1.

In certain embodiments, the porous base material is a non-woven fabricconsisting of natural or polymeric fibers. In some embodiments, thepolymeric fibers of the porous base material have a melting point of200° C. or higher. In certain embodiments, the polymeric fibers of theporous base material are selected from the group consisting ofpolyester, polyacetal, polyamide, polycarbonate, polyimide,polyetherether ketone, polyether sulfone, polyphenylene oxide,polyphenylene sulfide, polyethylene naphthalate, and combinationsthereof. In further embodiments, the polyester is polyethyleneterephthalate, polybutylene terephthalate, or a combination thereof.

In some embodiments, the organic binder is selected from the groupconsisting of polyester, polyamide, polyether, polyimide,polycarboxylate, polycarboxylic acid, polyvinyl compound, polyolefin,rubber, polyvinyl pyrrolidone, polyacrylic acid, polyacrylate,polymethacrylic acid, polymethacrylate, polystyrene, polyvinyl alcohol,polyvinyl acetate, polyacrylamide, cellulose, cellulose acetate,cellulose acetate butyrate, cellulose acetate propionate, carboxymethylcellulose, cyanoethylcellulose, cyanoethylsucrose, polyurethane, nitrilebutadiene rubber (NBR), styrene butadiene rubber (SBR), latex,acrylonitrile-styrene-butadiene copolymer, fluorinated polymer,chlorinated polymer, and combinations thereof.

In certain embodiments, the weight ratio of the inorganic filler to theorganic binder is from about 99:1 to about 1:1.

In some embodiments, the separator has a thickness from about 1 μm toabout 80 μm.

In certain embodiments, the difference in tensile strength of theseparator along the TD direction and MD direction is about 5% or less.

In another aspect, provided herein is a method for producing a battery,comprising, inserting the separator disclosed herein into the battery.

Also provided herein is a lithium battery comprising the separatordisclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic structure of a separator of Example 1.

FIG. 2 depicts a schematic structure of a separator of Example 3.

FIG. 3 depicts the cycling performance of a full lithium-ion batterycell having a separator of Example 1.

FIG. 4 depicts the cycling performance of a full lithium-ion batterycell having a separator of Example 2.

DETAILED DESCRIPTION OF THE INVENTION General Definitions

The term “porous base material” refers to a substrate having pores orvoids inside. The material used as a component of the porous basematerial may be an organic material or an inorganic material as long asthe material is an electrically insulating material. Any porous basematerial that has an electrically insulating property can be usedherein. Some non-limiting examples of the porous base material include aporous sheet formed of a fibrous material, such as woven or nonwovenfabric or a paper-like sheet. Some non-limiting examples of the fibrousmaterial include natural and polymeric fibers.

The term “non-woven” refers to products made by processes that do notinclude weaving nor knitting. The fibers in these materials are bondedtogether by chemical, mechanical, heat or solvent treatment.

The term “polymer” refers to a polymeric compound prepared bypolymerizing monomers, whether of the same or a different type. Thegeneric term “polymer” embraces the terms “homopolymer,” “copolymer,”“terpolymer” as well as “interpolymer.”

The term “interpolymer” refers to a polymer prepared by thepolymerization of at least two different types of monomers. The genericterm “interpolymer” includes the term “copolymer” (which generallyrefers to a polymer prepared from two different monomers) as well as theterm “terpolymer” (which generally refers to a polymer prepared fromthree different types of monomers). It also encompasses polymers made bypolymerizing four or more types of monomers.

The term “polyester” refers to a polymer having an ester functionalgroup in each repeating unit on its main chain. Some non-limitingexamples of suitable polyester include polyethylene terephthalate (PET),polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), andpoly(cyclohexanedimethylene terephthalate) (PCT).

The term “protective porous layer” refers to one or more layers coatedon one side or both sides of a porous base material. The protectiveporous layer includes a mixture of at least one organic binder and atleast one inorganic filler. The protective porous layer may furthercomprise one or more additives, in addition to the organic binder andinorganic filler. The protective porous layer may have a single-layered,two-layered, or multi-layered structure.

The term “organic binder” refers to a substance used for joining aninorganic filler to a porous base material or to each other. Any organicbinder that can join the inorganic filler to the porous base material orto each other can be used herein. Some non-limiting examples of theorganic binder include polyester, polyamide, polyacrylic acid,polyether, polyimide, polyolefin, rubber, styrene-butadiene rubber(SBR), acrylonitrile-butadiene rubber, cellulose, cellulose derivative,latex, and combinations thereof.

The term “unsaturated polymer” refers to a polymer having one or moreunsaturated groups in the polymer chain. The unsaturated groups can be,for example, carbon-carbon double or triple bonds or carbon-nitrogendouble or triple bonds.

The term “conjugated diene polymer” refers to a homopolymer of aconjugated diene, copolymer of two or more different conjugated dienesand copolymer of a conjugated diene and a vinyl-substituted aromatichydrocarbon. Some non-limiting examples of the conjugated diene polymerinclude polybutadiene, polyisoprene, butadiene/styrene random copolymer,isoprene/styrene random copolymer, acrylonitrile/butadiene copolymer,acrylonitrile/butadiene/styrene copolymer, butadiene/styrene blockcopolymer, styrene/butadiene/styrene block copolymer, isoprene/styreneblock copolymer, and styrene/isoprene/styrene block copolymer.

The term “block copolymer” refers to a polymer including two or morepolymer blocks.

The term “polymer block” refers to a grouping of multiple monomer units,which can be the same (e.g., a homopolymer block) or different (e.g., acopolymer block, a random copolymer block, etc.), and which are part ofa continuous polymer chain, which forms part of a larger polymer. A widevariety of block copolymers is contemplated herein including diblockcopolymers (i.e., polymers including two polymer blocks), triblockcopolymers (i.e., polymers including three polymer blocks), multiblockcopolymers (i.e., polymers including more than three polymer blocks),and combinations thereof.

The term “inorganic filler” refers to a substance that is electricallynon-conductive. Some non-limiting examples of the inorganic fillerinclude metal oxides, and non-oxide and non-metallic materials. Somenon-limiting examples of the metal oxides include aluminium oxides,zirconium oxides, barium titanate, lead zirconate titanates, ferrites,zinc oxide, and combinations thereof. Some non-limiting examples of thenon-oxide and non-metallic materials include silicon carbide, siliconnitride, aluminium nitride, boron nitride, titanium boride, molybdenumsilicide, and combinations thereof.

The term “whisker-type material” refers to an inorganic material in aneedle-like form. In some embodiment, the needle-like form has a regularshape, such as circular or polygonal shape. In certain embodiment, theneedle-like form has a uniform thickness along its length. In otherembodiment, the diameter of the whisker varies along its length, and theshape of the whisker may be irregular with alternating thick and thinsections.

The term “whisker” refers to a particle having an aspect ratio (i.e.length to diameter ratio) of about 5 or more.

The term “diameter of a whisker-type material” refers to a maximumcross-sectional dimension of a whisker.

The term “tensile strength” refers to the maximum stress a materialsubjected to a stretching load can withstand without tearing.

The term “machine direction” or “MD direction” refers to the directionalong the length of the separator. The MD direction is also referred toas a “longitudinal direction.”

The term “transverse direction” or “TD direction” refers to thedirection across the separator or perpendicular to the machinedirection. The TD direction is also referred to as a “width direction.”

The term “first layer of the protective porous layer” refers to a layeradjacent to a porous base material.

The term “second layer of the protective porous layer” refers to a layeron and in contact with the first layer of the protective porous layer.

The term “substantial portion” of an inorganic filler refers to aportion greater than 99.9%, greater than 99.5%, greater than 99%,greater than 98%, greater than 97%, greater than 96%, greater than 95%,greater than 90%, greater than 85%, greater than 80%, greater than 75%,greater than 70%, greater than 65%, greater than 60%, greater than 55%,or greater than 50%, based on the total volume or weight of theinorganic filler.

The term “first side of the protective porous layer” refers to a sideadjacent to a porous base material.

The term “second side of the protective porous layer” refers to a sideadjacent to an anode or a cathode.

The term “porosity” refers to the total void space in a materialattributable to the presence of pores, or the ratio of the pore volumeto the total volume of a material. In the case where the separator ismade up of a porous base material and a protective porous layer, thenumber of pores of the separator is the sum of the number of pores ofthe porous base material and the number of pores of the protectiveporous layer.

The term “pore volume” refers to the total void space in a materialattributable to the presence of pores in units of volume percent.

The term “average pore size” refers to a value calculated by a formula4V/S where S is a specific surface area and V is a pore volume per unitmass obtained from a pore size distribution measured by a mercurycompression method.

The term “cylindrical pore” or “cylindrically shaped pore” refers to apore formed through the top and the bottom of the separator, of which across section is circular or nearly circular on both the top and thebottom, and each cross section at the top, bottom and passage has thesame or similar size.

The term “heat resistance” refers to a characteristic in which meltingor decomposition does not occur in a temperature range of 200° C. orlower.

The term “C rate” refers to the charging or discharging rate of a cellor battery, expressed in terms of its total storage capacity in Ah ormAh. For example, a rate of 1 C means utilization of all of the storedenergy in one hour; a 0.1 C means utilization of 10% of the energy inone hour and the full energy in 10 hours; and a 5 C means utilization ofthe full energy in 12 minutes.

The term “ampere-hour (Ah)” refers to a unit used in specifying thestorage capacity of a battery. For example, a battery with 1 Ah capacitycan supply a current of one ampere for one hour or 0.5 A for two hours,etc. Therefore, 1 Ampere-hour (Ah) is the equivalent of 3600 coulombs ofelectrical charge. Similarly, the term “miniampere-hour (mAh)” alsorefers to a unit of the storage capacity of a battery and is 1/1,000 ofan ampere-hour.

The term “doctor blading” refers to a process for fabrication of largearea films on rigid or flexible substrates. A coating thickness can becontrolled by an adjustable gap width between a coating blade andcoating surface, which allows the deposition of variable wet layerthicknesses.

In the following description, all numbers disclosed herein areapproximate values, regardless whether the word “about” or “approximate”is used in connection therewith. They may vary by 1 percent, 2 percent,5 percent, or, sometimes, 10 to 20 percent. Whenever a numerical rangewith a lower limit, R^(L), and an upper limit, R^(U), is disclosed, anynumber falling within the range is specifically disclosed. Inparticular, the following numbers within the range are specificallydisclosed: R=R^(L)+k*(R^(U)−R^(L)), wherein k is a variable ranging from1 percent to 100 percent with a 1 percent increment, i.e., k is 1percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50 percent,51 percent, 52 percent, . . . , 95 percent, 96 percent, 97 percent, 98percent, 99 percent, or 100 percent. Moreover, any numerical rangedefined by two R numbers as defined in the above is also specificallydisclosed.

The present invention provides a secondary-battery separator comprising,a porous base material, and a protective porous layer coated on one orboth surfaces of the porous base material, wherein the protective porouslayer comprises an organic binder and an inorganic filler, and whereinthe inorganic filler comprises a whisker-type material selected from thegroup consisting of Al₂O₃, SiO₂, TiO₂, ZrO₂, BaO_(x), ZnO, CaCO₃, TiN,AlN, MTiO₃, K₂O.nTiO₂, Na₂O.mTiO₂, and combinations thereof, wherein xis 1 or 2; M is Ba, Sr or Ca; n is 1, 2, 4, 6 or 8; and m is 3 or 6.

The separator disclosed herein is suitable for primary and secondary(rechargeable) lithium batteries, for nickel metal hydride,nickel-cadmium, and silver-zinc batteries. It is also suitable for usein battery systems with comparatively high operating temperatures.

Generally, conventional organic separator may comprise a porous basematerial and a protective porous layer. However, all of the conventionalseparators have never possessed an important relation between the porousbase material and inorganic filler which enables strong tensile strengthof the separator along the TD direction and MD direction. In particular,no prior art document describes a protective porous layer comprising aninorganic filler in the form of whisker. Separators disclosed hereinhave high porosity, uniform pore distribution, well defined andcontrolled pore geometry, and high film strength.

Furthermore, the present invention does not follow in obvious manner ofthe choice of the porous base material and inorganic filler. The presentinvention therefore overcomes the problems associated with prior arts asmentioned above.

The separator of the present invention also has a distinct advantageover separators based on woven or non-woven polymeric or ceramic fabricsas described in prior art. One reason for this is that the porous basematerial disclosed herein exhibits a high and similar tensile strengthin both TD and MD directions and hence a high tearing resistance.

The porous base material may comprise woven or nonwoven polymericfibers, natural fibers, carbon fibers, glass fibers or ceramic fibers.In some embodiments, the porous base material comprises woven ornonwoven polymeric fibers.

In certain embodiment, the porous base material is a nonwoven comprisingpolymeric fibers. In some embodiment, the fibers of the nonwoven couldbe made of organic polymers, such as polyolefin, polyester, polyacetal,polyamide, polycarbonate, polyimide, polyetherether ketone,polysulfones, polyphenylene oxide, polyphenylene sulfide,polyacrylonitrile, polyvinylidene fluoride, polyoxymethylene, polyvinylpyrrolidone, or a combination thereof. But all other known polymericfibers or many natural fibers can be used as well.

Some non-limiting examples of suitable polyolefin include polypropylene,polyethylene, and polypropylene/polyethylene co-polymer.

In order to improve thermal stability of the nonwoven, the fibers havinga melting temperature of 200° C. or above should be used. In someembodiment, the fibers are selected from polyester. Some non-limitingexamples of suitable polyester include polyethylene terephthalate,polybutylene terephthalate, polyethylene naphthalate, polybutylenenaphthalate, derivatives thereof, and combinations thereof. The use ofthese organic polymers allows the production of a separator thatexhibits only a small amount of thermal shrinkage.

The nonwoven fabric may be produced by a publicly known process. Somenon-limiting examples of suitable process include dry process, spun bondprocess, water needle process, spun lace process, wet process,melt-blowing process and the like.

In some embodiment, the natural or polymeric fibers of the porous basematerial have an average thickness from about 0.5 μm to about 10 μm,from about 1 μm to about 10 μm, from about 1.5 μm to about 10 μm, fromabout 1 μm to about 7 μm, from about 1 μm to about 5 μm, or from about2.5 μm to about 7.5 μm. In certain embodiments, the fibers of the porousbase material have an average thickness from about 0.2 μm to about 10μm.

If the average thickness of fibers is less than 0.2 μm, the non-wovenfabric produced has deteriorated mechanical properties. Also, if theaverage thickness of fibers is greater than 10 μm, it is not easy tocontrol the size of pores in the non-woven fabric.

In some embodiments, the porous base material has a thickness from about10 μm to about 200 μm, from about 30 μm to about 100 μm, from about 15μm to about 80 μm, from about 25 μm to about 50 μm, from about 15 μm toabout 40 μm, from about 15 μm to about 30 μm, from about 15 μm to about25 μm, from about 15 μm to about 20 μm, or from about 10 μm to about 20μm.

The thickness of the porous base material has a considerable influenceon the properties of the separator. Thinner separators allow anincreased packing density in a battery pack since a larger amount ofenergy can be stored in the same volume.

In some embodiments, the porous base material has a thickness of about15 μm, about 20 μm, or about 25 μm. Separators having such a thicknessmake it possible to build very compact batteries with a high energydensity.

The separator of the present invention includes a planar nonwoven fabricbase material having a plurality of pores.

In some embodiment, the porous base material has an average pore sizefrom about 10 nm to about 2000 nm, from about 150 nm to about 1500 nm,from about 20 nm to about 1000 nm, from about 300 nm to about 1000 nm,from about 300 nm to about 800 nm, or from about 300 nm to about 500 nm.

Porosity of the porous base material in this context is defined as thevolume of the nonwoven (100%) minus the volume of the fibers of thenonwoven, i.e. the fraction of the volume of the nonwoven that is nottaken up by material. The volume of the nonwoven can be calculated fromthe dimensions of the nonwoven. The volume of the fibers is calculatedfrom the measured weight of the nonwoven in question and the density ofthe polymeric fibers.

In some embodiment, the porous base material has a porosity from about50% to about 97%, from about 50% to about 95%, from about 50% to about80%, from about 55% to about 90%, from about 55% to about 80%, fromabout 60% to about 95%, from about 60% to about 90%, from about 60% toabout 80%, from about 65% to about 90%, from about 65% to about 80%,from about 70% to about 90%, from about 70% to about 80%, from about 75%to about 90%, or from about 80% to about 90%.

The nature of the porous base material disclosed herein, which comprisesa particularly useful combination of thickness and porosity, makes itpossible to produce separators which meet the requirements forseparators in high power batteries, especially lithium high powerbatteries.

In certain embodiments, the protective porous layer comprises an organicbinder and an inorganic filler. In some embodiments, the inorganicfiller does not comprise a particulate-type material and is awhisker-type material.

The whisker-type material has a needle-like, acicular shape, which has ahigh elastic modulus. Some non-limiting examples of the whisker-typematerial include inorganic oxides, carbides, borides and nitrides. Insome embodiments, the whisker-type material is a mixture of whiskers ofdifferent materials.

In some embodiments, the whisker-type material in the protective porouslayer is randomly oriented, forming a network structure. The use of thewhisker-type material provides great improvements in the mechanicalproperties of the separator over particulate-type material in both theTD direction and MD direction. Hence, it improves the fracturetoughness, hardness and strength of the separator. The protective porouslayer containing the whisker-type material disclosed herein alsodramatically improves abrasion resistance of the separator.

FIG. 1 shows a schematic structure of a separator (1) of Example 1, inwhich a protective porous layer (2) having a single-layered structurecomprising a whisker-type material (3) and an organic binder (4) isformed on a porous base material (5), wherein the whisker-type materialis randomly distributed throughout the protective porous layer.

In some embodiments, the whisker-type material has an average diameterfrom about 0.05 μm to about 3 μm, from about 0.1 μm to about 3 μm, fromabout 0.5 μm to about 3 μm, from about 0.5 μm to about 2 μm, from about0.5 μm to about 1 μm, from about 0.6 μm to about 2 μm, or from about 0.8μm to about 2 μm.

In some embodiments, the whisker-type material has an average lengthfrom about 0.05 μm to about 30 μm, from about 0.05 μm to about 25 μm,from about 0.05 μm to about 20 μm, from about 0.1 μm to about 20 μm,from about 1 μm to about 20 μm, from about 1 μm to about 15 μm, fromabout 1 μm to about 10 μm, from about 1 μm to about 8 μm, from about 1μm to about 5 μm, from about 2 μm to about 10 μm, from about 2 μm toabout 8 μm, from about 3 μm to about 10 μm, from about 4 μm to about 20μm, from about 4 μm to about 10 μm, from about 5 μm to about 20 μm, orfrom about 6 μm to about 15 μm.

In certain embodiments, the whisker-type material has a ratio of lengthto diameter from about 5 to about 300, from about 5 to about 250, fromabout 5 to about 200, from about 5 to about 150, from about 5 to about100, from about 6 to about 20, from about 6 to about 13, from about 6 toabout 10, from about 7.5 to about 20, from about 25 to about 300, fromabout 25 to about 200, from about 25 to about 100, from about 50 toabout 300, from about 50 to about 200, or from about 50 to about 100. Insome embodiments, the whisker-type material has a ratio of length todiameter of about 5:1, about 8:1, about 10:1, about 15:1, about 20:1,about 50:1, about 100:1, or about 150:1.

The concomitant use of two types of inorganic fillers allows us tocontrol the pore size and porosity of the separator by varying thediameter and ratio of the whisker-type material and particulate-typematerial.

In some embodiments, the inorganic filler further comprises aparticulate-type material selected from the group consisting of Al₂O₃,SiO₂, TiO₂, ZrO₂, BaO_(x), ZnO, CaCO₃, TiN, AlN, and combinationsthereof, wherein x is 1 or 2.

In certain embodiments, the particulate-type material has an averagediameter from about 100 nm to about 10 μm, from about 100 nm to about2000 nm, from about 100 nm to about 1000 nm, from about 250 nm to about1500 nm, from about 300 nm to about 3 μm, from about 500 nm to about 4.5μm, from about 500 nm to about 6 μm, from about 1 μm to about 20 μm,from about 1 μm to about 10 μm, from about 10 μm to about 20 μm, fromabout 1 μm to about 15 μm, from about 1 μm to about 7.5 μm, from about 1μm to about 4.5 μm, from about 1 μm to about 3 μm, or from about 800 nmto about 2.5 μm.

In some embodiments, the protective porous layer comprising aparticulate-type material and a whisker-type material has asingle-layered structure, wherein the particulate-type material and thewhisker-type material are randomly distributed throughout the protectiveporous layer. In certain embodiments, the whisker-type material and theparticulate-type material can be stably fixed by an organic binder onthe surface of the porous base material. The particulate-type materialcan also be fixed in a part of the pores of the porous base material.

In certain embodiments, the protective porous layer comprising aparticulate-type material and a whisker-type material has a two-layeredstructure comprising a first layer and a second layer, wherein the firstlayer is adjacent to the porous base material and the second layer is onand in contact with the first layer. In some embodiments, theparticulate-type material resides in the first layer and thewhisker-type material resides in the second layer. In other embodiments,a substantial portion of the particulate-type material resides in thefirst layer and a substantial portion of the whisker-type materialresides in the second layer. The particulate-type material can be stablyfixed by an organic binder on the surface of the porous base material,and in a part of the pores of the porous base material. The whisker-typematerial can be stably fixed by an organic binder on the surface of thefirst layer.

FIG. 2 shows a schematic structure of a separator (6) of Example 3, inwhich a protective porous layer (7) having a two-layered structurecomprising a whisker-type material (3), a particular-type material (8),and an organic binder (4) is formed on a porous base material (5),wherein the whisker-type material is randomly distributed throughout thesecond layer of the protective porous layer.

The particulate-type material serves not only to control the pore sizeand porosity of the separator but also maintains the physical shape ofthe protective porous layer. The whisker-type material can significantlyincrease the mechanical strength of the protective porous layer, therebyimproving the tensile strength and impact resistance of the separator.It is because a network structure is obtained by random orientation anddistribution of the whisker-type material in the protective porouslayer. Hence, the use of the whisker-type material enables reinforcementof the mechanical strength of the separator.

The process for making battery separators generally leads to anisotropicnature of battery separators. For example, the mechanical properties ofthe separator are anisotropic. The tensile strength in the TD directionis relatively low. The anisotropic nature of battery separators has beendescribed in Ralph J. Brodd, “Batteries for Sustainability,” Springer,Chapter 6, 2012, p 145-148, which is incorporated herein by reference.

When there is a substantial difference between the MD tensile strengthand the TD tensile strength, the separator suffers from deterioration instability, for example thermal resistance and physical strength such aspuncture strength of the separator. Therefore, it is another objectiveof the present invention to provide a separator that has improvedstability by obtaining similar tensile strength in both the MD and TDdirections.

The use of whisker-type material allows us to control the tensilestrength in both the MD and TD directions by varying the diameter, ratioof length to diameter, and orientation of the whisker-type material inmanufacture of the separator regardless of the anisotropic nature of theporous base material.

In some embodiments, the whisker-type material in the protective porouslayer is unidirectionally oriented in either the TD direction or the MDdirection. In certain embodiments, the whisker-type material tends to beoriented and aligned in the TD direction. In some embodiments, at least50% of the whisker-type material are lying within an angle of ±35°,±30°, ±25°, ±20°, ±15°, or ±10° about an axis in the TD direction.

The separator disclosed herein exhibits similar tensile strengths inboth the MD and TD directions. In some embodiment, a difference intensile strength of separator along the TD direction and MD direction isabout 30% or less, about 25% or less, about 20% or less, about 18% orless, about 15% or less, about 13% or less, about 10% or less, about 8%or less, about 7% or less, about 6% or less, about 5% or less, about 4%or less, about 3% or less, about 2% or less, or about 1% or less. Aseparator having similar tensile strengths along the TD direction andthe MD direction can prevent the separator from being damaged duringassembly into battery.

The protective porous layer made of an inorganic filler that is boundvia the organic binder has a structure wherein void space is formedbetween the inorganic filler. This void space constitutes pores of theprotective porous layer.

The protective porous layer disclosed herein also achieves aparticularly high porosity with the pores being sufficiently small toprevent lithium dendrites growing through the separator.

The protective porous layer of the present invention has uniform porestructure allowing lithium ions to move smoothly therethrough.

In addition, since pores present in the particulate-type material of thepresent invention have such a size that lithium ions solvated withelectrolyte solvent molecules can sufficiently pass therethrough, theycan serve as an additional pathway for lithium ions. Therefore, lithiumion conductivity in a battery is improved.

Moreover, the inorganic filler disclosed herein has high dielectricconstant, which can contribute to increase the dissociation degree of anelectrolyte salt in a liquid electrolyte, such as a lithium salt,thereby ensuring sufficient ionic conductivity.

There is no particular limitation in thickness of the protective porouslayer of the present invention. In some embodiments, the protectiveporous layer has a thickness from about 1 μm to about 100 μm, from about1 μm to about 50 μm, from about 2 μm to about 30 μm, from about 2 μm toabout 25 μm, from about 4 μm to about 20 μm, from about 4 μm to about 15μm, from about 5 μm to about 25 μm, from about 5 μm to about 20 μm, fromabout 5 μm to about 15 μm, or from about 5 μm to about 10 μm.

The coating solution for a separator comprising an inorganic filler,organic binder, and solvent with desired ratio is coated onto at leastone surface of a porous base material. The thickness of the protectiveporous layer coated onto the porous base material will depend upon theparticular composition of the coating solution and the final thicknessdesired in the separator.

In some embodiments, the average thickness ratio of the second layer ofthe protective porous layer to the first layer of the protective porouslayer is from about 1:3 to about 3:1.

In certain embodiments, the average thickness ratio of the second layerof the protective porous layer to the first layer of the protectiveporous layer is about 1:3, about 1:2.5, about 1:2, about 1:1.5, about1:1.3, about 1:1, about 1.3:1, about 1.5:1, about 2:1, about 2.5:1, orabout 3:1.

Another advantage of the separator disclosed herein is that it hasoutstanding safety and exhibits no or very slight contraction at hightemperature. It is because the inorganic filler which adheres to theporous base material has a melting point which is well above thesafety-relevant temperature range for electrochemical cells and hencesuppresses thermal contraction of the separator. In particular, anetwork structure is obtained by random orientation and distribution ofthe whisker-type material in the protective porous layer, whichcontributes to a superior resistance to thermal shrinkage.

A variety of different coating and printing methods can be used to coator print a protective porous layer onto a porous base material. Anymethods that can deposit a coating solution comprising an inorganicfiller and organic binder onto the porous base material can be usedherein. Some non-limiting example of the methods include doctor blading,slot-die coating, gravure coating, and screen printing.

In some embodiments, the coating method is slot-die coating. In general,a coating solution is forced out from a reservoir through a slot bypressure, and transferred to a moving substrate. The slot die can be ina horizontal position orientation or downward facing orientation. Duringthe coating process, the whiskers may tend to be oriented in the TDdirection.

In certain embodiments, the coating method is gravure coating. Gravurecoating differs from many of the conventional roll coating techniques inthat one of the rolls is patterned with a surface engraving the gravurepattern. Both the shape and the size of the gravure pattern can bevaried which affects the final properties of the coating. In general,there are two types of common variants. They are direct gravure coatingand offset gravure coating. Direct gravure coating is where the fluidtransfer takes place directly from the gravure roll to the substrate. Inoffset gravure coating, fluid is transferred first from the gravure rollto a smooth deformable roll (often termed the applicator roll), and thenfrom the deformable roll to the substrate. During the coating process,the whiskers may tend to be oriented in the TD direction.

In some embodiments, the printing method is screen printing. In general,screen printing is a printing technique whereby a screen having a meshsize defining flow spaces for said printing medium. In certainembodiments, the flow spaces are rectangular shape and larger than thesize of the whisker-type material. During the coating process, thewhisker-type material may tend to be oriented in the TD direction or theMD direction depending on the orientation of the screen.

In some embodiments, the protective porous layer of the presentinvention contains an inorganic filler and an organic binder. Theinorganic filler could be joined to the nonwoven or to each other by theorganic binder. In certain embodiments, the organic binder is an organicpolymer. The use of the organic polymer makes it possible to produce aseparator with adequate mechanical flexibility.

Some non-limiting examples of the organic binder include polyester,polyamide, polyether, polycarboxylates, polycarboxylic acid, polyvinylcompound, polyolefin, rubber, polyvinyl pyrrolidone, polyacrylic acid,polyacrylate, polymethacrylic acid, polymethacrylate, polystyrene,polyvinyl alcohol, polyvinyl acetate, polyacrylamide, cellulose,cellulose acetate, cellulose acetate butyrate, cellulose acetatepropionate, carboxymethyl cellulose, cyanoethylcellulose,cyanoethylsucrose, polyurethane, nitrile butadiene rubber (NBR), styrenebutadiene rubber (SBR), latex, acrylonitrile-styrene-butadienecopolymer, halogenated polymer, fluorinated polymer, chlorinatedpolymer, unsaturated polymer, conjugated diene polymer, and combinationsthereof.

Some non-limiting examples of the polyvinyl compound include those thatconsist of N-vinylamide monomers such as N-vinyl formamide and N-vinylacetamide or that contain these monomers. The poly-N-vinyl compound ischaracterized by good wettability. Homopolymers, copolymers, and blockcopolymers can also be used herein. In some embodiments, the polyvinylcompound is a random, block or alternating interpolymer. In furtherembodiments, the polyvinyl compound is a di-block, tri-block or othermulti-block interpolymer.

Some non-limiting examples of the rubber include natural rubber,isoprene rubber, butadiene rubber, chloroprene rubber, styrene butadienerubber, and nitrile butadiene rubber. These rubbers contain unsaturateddouble bonds. In some embodiments, the rubber is a random, block oralternating interpolymer. In further embodiments, the rubber is adi-block, tri-block or other multi-block interpolymer. Unsaturatedpolymers are generally characterized by good adhesive properties.

In some embodiment, the organic binder is a water-soluble polymer. Incertain embodiment, the water-soluble polymer is a monomer containing acarboxylic acid group, a sulfonic acid group, or a combination thereof.

Some non-limiting examples of the monomer having a carboxylic acid groupinclude monocarboxylic acid, dicarboxylic acid, anhydride ofdicarboxylic acid, and derivatives thereof. Some non-limiting examplesof the monocarboxylic acid include acrylic acid, methacrylic acid,crotonic acid, 2-ethylacrylic acid, and isocrotonic acid. Somenon-limiting examples of the dicarboxylic acid include maleic acid,fumaric acid, itaconic acid, and methyl maleic acid. Some non-limitingexamples of the anhydride of dicarboxylic acid include maleic anhydride,acrylic anhydride, methyl maleic anhydride, and dimethyl maleicanhydride.

Some non-limiting examples of the monomer having a sulfonic acid groupinclude vinylsulfonic acid, methyl vinylsulfonic acid,(meth)allylsulfonic acid, styrenesulfonic acid, (meth)acrylicacid-2-ethyl sulfonate, 2-acrylamide-2-methylpropanesulfonic acid,3-allyloxy-2-hydroxypropanesulfonic acid, and2-(N-acryloyl)amino-2-methyl-1,3-propane-disulfonic acid.

By varying the concentration of the organic binder and inorganic fillerin the slurry, control over the homogeneity of the organic binder can beobtained. In certain embodiments, the amount of the organic binder isdistributed homogeneously throughout the protective porous layer.

In some embodiments, the protective porous layer is a single-layeredstructure and has a first side adjacent to the porous base material anda second side adjacent to an anode or a cathode, wherein an amount ofthe organic binder at the first side is greater than an amount of theorganic binder at the second side.

In certain embodiments, the protective porous layer is a two-layeredstructure comprising a first layer and a second layer, wherein the firstlayer has a first side adjacent to the porous base material and thesecond layer has a second side adjacent to an anode or a cathode,wherein an amount of the organic binder at the first side is greaterthan an amount of the organic binder at the second side.

When more organic binder is included at the first side than at thesecond side, the separator may be better adhered to the porous basematerial. Hence, the separator disclosed herein has good peelingresistance.

There is no particular limitation in mixing ratio of an inorganic fillerto an organic binder in the protective porous layer of the presentinvention. The mixing ratio of the inorganic filler to the organicbinder can be controlled according to the thickness and structure of theprotective porous layer to be formed.

In some embodiment, a weight ratio of the inorganic filler to theorganic binder in the protective porous layer formed on the porous basematerial according to the present invention is from about 1:1 to about99:1, from about 70:30 to about 95:5, from about 95:5 to about 35:65,from about 65:35 to about 45:55, from about 20:80 to about 99:1, fromabout 10:90 to about 99:1, from about 5:95 to about 99:1, from about3:97 to about 99:1, from about 1:99 to about 99:1, or from about 1:99 toabout 1:1.

If the weight ratio of the inorganic filler to the organic binder isless than 1:99, the content of binder is so great that pore size andporosity of the protective porous layer may be decreased. When thecontent of the inorganic filler is greater than 99 wt. %, the polymercontent is too low to provide sufficient adhesion among the inorganicfiller, resulting in degradation in mechanical properties and impairedpeeling resistance of a finally formed protective porous layer.

In some embodiments, the protective porous layer disclosed herein maycomprise at least one additive other than the inorganic filler andorganic binder for the purposes of improving and/or controlling theprocessibility, physical, chemical, and/or mechanical properties of theseparator. Some non-limiting examples of the additive include aviscosity modifier, a surfactant, a defoaming agent, or a combinationthereof. The additive is generally less than 2% by weight, or from about0.1% to about 1% by weight, based on the total weight of the inorganicfiller and organic binder.

A coating slurry added with the viscosity modifier can have highstability and low tendency to cause sedimentation or aggregation ofsolid content.

Some non-limiting examples of the viscosity modifier include cellulosederivatives such as carboxymethyl cellulose (CMC); poly(meth)acrylicacid salts such as sodium poly(meth)acrylate; polyvinyl alcohol,modified polyvinyl alcohol, and polyethylene oxide;polyvinylpyrrolidone, polycarboxylic acid, oxidized starch, starchphosphate, casein, modified starches, and chitin and chitosanderivatives.

Some non-limiting examples of the cellulose derivatives includecarboxymethyl cellulose, carboxymethylethyl cellulose, methyl cellulose,ethyl cellulose, ethylhydroxyethyl cellulose, hydroxyethyl cellulose,hydroxypropyl cellulose, and ammonium salts and alkali metal saltsthereof.

A surfactant can be used to improve the homogeneity and stability of thecoating slurry and thus coating of the separator. This leads to theimproved mechanical properties of the separator.

In some embodiment, the surfactant is a non-ionic surfactant. Somenon-limiting examples of suitable non-ionic surfactant includealkoxylated alcohol, carboxylic ester, polyethylene glycol ester, andcombinations thereof. Some non-limiting examples of suitable alkoxylatedalcohol include ethoxylated and propoxylated alcohols. Some non-limitingexamples of suitable ethoxylated alcohols include octylphenolethoxylates and nonylphenol ethoxylates.

The separator disclosed herein exhibits a high porosity. The pore sizeand porosity of the separator are mainly dependent on the size and shapeof the inorganic filler. In some embodiment, the separator of thepresent invention has a porosity from about 30% to about 99%, from about40% to about 95%, from about 50% to about 95%, from about 50% to about90%, from about 60% to about 90%, from about 70% to about 90%, fromabout 40% to about 75%, from about 20% to about 60%, from about 35% toabout 60%, from about 45% to about 55%, or from about 30% to about 50%.

In some embodiment, the separator of the present invention has aporosity of more than 40%, more than 50%, more than 55%, more than 60%,more than 65%, more than 70%, more than 75%, more than 80%, more than85%, more than 90%, or more than 95%. A separator having this porositycan yield a battery with a good ion permeability.

The large porosity of the substrate means a higher porosity for theseparator of the present invention, which is why a higher uptake ofelectrolytes is obtainable with the separator of the present invention.

In some embodiment, the separator of the present invention has anaverage pore size from about 1 nm to about 350 nm, from about 1 nm toabout 100 nm, from about 20 nm to about 100 nm, from about 40 nm toabout 350 nm, from about 40 nm to about 80 nm, from about 50 nm to about80 nm, from about 0.1 μm to about 70 μm, from about 1 μm to about 60 μm,from about 1 μm to about 50 μm, from about 1 μm to about 40 μm, fromabout 1 μm to about 30 μm, from about 1 μm to about 20 μm, from about 1μm to about 10 μm, from about 1 μm to about 5 μm, or from about 1 μm toabout 3 μm.

Separator of the present invention prevents short circuits due to metaldendrite growth. No dendrite penetration can form from one side to theother side of the separator. Contrary to cylindrical pores of aseparator, it is conceivable that the pores might form a labyrinthinestructure in which no dendrite penetration can form from one side to theother side of the separator. Therefore, there is no need to increase thethickness of the separator for preventing dendrite penetration. Aseparator that is too thick can compromise battery capacity.

In some embodiments, the separator has a thickness from about 10 μm toabout 200 μm, from about 30 μm to about 100 μm, from about 10 μm toabout 75 μm, from about 10 μm to about 50 μm, from about 10 μm to about20 μm, from about 15 μm to about 40 μm, from about 15 μm to about 35 μm,from about 20 μm to about 40 μm, from about 20 μm to about 35 μm, fromabout 20 μm to about 30 μm, from about 30 μm to about 60 μm, from about30 μm to about 50 μm, or from about 30 μm to about 40 μm.

In some embodiment, the separator of the present invention has athickness of less than 40 μm, less than 35 μm, less than 30 μm, lessthan 25 μm, or less than 20 μm. A separator having this thickness canyield a battery with a high power density.

In another aspect, provided herein is a method for producing a battery,comprising, inserting the separator disclosed herein into the battery.

Also provided herein is a lithium battery comprising the separatordisclosed herein.

The separators according to the present invention are highly suitablefor use in fast charging batteries. A battery equipped with thisseparator is heat-resistant, and is therefore able to tolerate thetemperature increase due to the rapid charging without adverse changesto the separator and/or damages to the battery. These batteriesconsequently can have a very rapid charging rate.

This is a distinct advantage when batteries equipped with the separatordisclosed herein are used in electric vehicles or hybrid vehicles, sincefast charging is feasible in the course of an hour or less.

The nail penetration test is an important method to access the safety ofLi-ion cells presumably to simulate internal shorts, which is widelyused across the battery industry and battery-user community. It involvesdriving a metallic nail through a charged Li-ion cell at a prescribedspeed. The cell is deemed to have passed if there is no smoke or flamefollowing the puncturing, by visually confirmation.

In the nail penetration test, a nail was used to puncture a battery. Ashort circuit current flows through the nail and causes a thermalrunaway with a sharp temperature rise. The separator disclosed hereindoes not exhibit a large degree of shrinkage. Even though the polymer isat or above its melting point or partially dissolved, the structure ofthe protective porous layer does not collapse. Only the polymeric porousbase material would slightly melt at the site of puncture and contractbut not the inorganic filler. Therefore, the short circuit location doesnot further expand. The separator disclosed herein has a high stability.In this manner, the organic separator plays a role of keeping safety ofthe lithium ion secondary battery and this safety feature isparticularly important for automotive use.

The following examples are presented to exemplify embodiments of theinvention but are not intended to limit the invention to the specificembodiments set forth. Unless indicated to the contrary, all parts andpercentages are by weight. All numerical values are approximate. Whennumerical ranges are given, it should be understood that embodimentsoutside the stated ranges may still fall within the scope of theinvention. Specific details described in each example should not beconstrued as necessary features of the invention.

EXAMPLES

The tensile strengths of the separators (Examples 1-6) below weremeasured by a universal testing machine UTM (obtained from MTS SystemsCorporation, US; model no. MTS 895). Each of the separators was cut intoa rectangular shape having a size of 10 mm×50 mm (length (MD)×width(TD)) at 10 different regions to obtain 10 specimens. Then, each of thespecimens was placed in the machine between the grips and gripped tohave a length of 20 mm, followed by measurement of average tensilestrength in the machine direction (MD) and the transverse direction (TD)while applying pulling force to the specimen at room temperature. Thestress at break was taken as tensile strength.

The thickness of each separator was measured using a contact thicknessmeter (obtained from Sony Manufacturing Systems Corporation, JP; modelno. digital micrometer M-30).

The porosity can be determined by the known method of mercuryporosimetry in accordance with DIN 66133. Porosimetry is an analyticaltechnique used to determine various quantifiable aspects of a material'sporous nature. This technique measures the pressure required to forcemercury into membrane pores through the use of a porosimeter.

The nail penetration test was conducted by passing a stainless steelnail with 3 mm diameter and 80° taper angle through the test cell in astate of charge of 95% with a speed of 1 cm/second.

The chemicals were purchased and used as received.

Example 1

Preparation of a Separator Coated with a Whisker-Type Material

An aqueous binder solution was prepared by dissolving 50 g ofcarboxymethyl cellulose (CMC) (obtained from DAICEL Corporation, Japan;product no. CMC1390) in 6.55 L de-ionized water. To the aqueous bindersolution were added 125 g of CaCO₃ whiskers (obtained from Jiangxi NPNew Materials Technology Co. Ltd., China; product no. NP-CW2) and 7.5 gof styrene butadiene rubber (SBR) (obtained from NIPPON A&L INC., Japan;product no. AL-2001). The whiskers had an average diameter of 0.5 μm andan average length of 8 μm. After the addition, the suspension wasstirred for 40 minutes at room temperature at a stirring speed of 50 rpm(revolution per minute) to form a slurry.

A 30 cm wide nonwoven PET fabric (obtained from MITSUBISHI PAPER MILLSLTD, Japan) having a thickness of about 20 μm and a weight per unit areaof about 10 g/m² was then coated with the above slurry by a continuousroll coater having a doctor blade (obtained from Shenzhen KEJINGSTARTechnology Ltd., China; model no. AFA-EI300-UL). The nonwovensubsequently passed through an oven integrated in the roll coater anddried at a temperature of 100° C. in a hot air stream. The coating speedwas in the range of 1.2-1.5 meter/minute. A coating thickness wascontrolled by an adjustable gap width between a coating blade andcoating surface. A coated separator having a total thickness of about 28μm and a porosity of about 68% was obtained.

Example 2

Preparation of a Separator Coated with a Whisker-Type Material

An aqueous binder solution was prepared by dissolving 50 g of CMC in6.55 L de-ionized water. To the aqueous binder solution were added 105 gof ZnO whiskers (obtained from Hefei Aijia New Material TechnologyCompany Ltd., China) and 7.5 g of SBR. The whiskers had an averagediameter of 1 μm and an average length of 11 μm. After the addition, thesuspension was stirred for 55 minutes at room temperature at a stirringspeed of 50 rpm to form a slurry.

A 30 cm wide nonwoven PET fabric (obtained from MITSUBISHI PAPER MILLSLTD, Japan) having a thickness of about 20 μm and a weight per unit areaof about 10 g/m² was then coated with the above slurry by a continuousroll coater having a doctor blade. The nonwoven subsequently passedthrough an oven and dried at a temperature of 100° C. in a hot airstream. The coating speed was in the range of 1.2-1.5 meter/minute. Acoating thickness was controlled by an adjustable gap width between acoating blade and coating surface. A coated separator having a totalthickness of about 33 μm and a porosity of about 58% was obtained.

Example 3

Preparation of a Separator Coated with a Mixture of Whisker-Type andParticular-Type Materials

An aqueous binder solution was prepared by dissolving 55 g of CMC in6.55 L de-ionized water. To the aqueous binder solution were added 50 gof Al₂O₃ particles (obtained from Taimei Chemicals Co. Ltd., Japan;product no. TM-100), 65 g of K₂TiO₃ whiskers (obtained from ShanghaiDian Yang Industry Co. LTD, China), and 8.0 g of SBR. The particles hadan average diameter of 6 μm. The whiskers had an average diameter of 0.6μm and an average length of 12 μm. After the addition, the suspensionwas stirred for 60 minutes at room temperature at a stirring speed of 50rpm to form a slurry.

A 30 cm wide nonwoven PET fabric (obtained from MITSUBISHI PAPER MILLSLTD, Japan) having a thickness of about 20 μm and a weight per unit areaof about 10 g/m² was then coated with the above slurry by a continuousroll coater having a doctor blade. The nonwoven subsequently passedthrough an oven and dried at a temperature of 100° C. in a hot airstream. The coating speed was in the range of 1.5-1.8 meter/minute. Acoating thickness was controlled by an adjustable gap width between acoating blade and coating surface. A coated separator having a totalthickness of about 32 μm and a porosity of about 52% was obtained.

Example 4

Preparation of a Separator Coated with a Mixture of Whisker-Type andParticular-Type Materials

An aqueous binder solution was prepared by dissolving 60 g of CMC in6.55 L de-ionized water. To the aqueous binder solution were added 40 gof TiO₂ particles (obtained from Shanghai Dian Yang Industry Co. LTD,China), 75 g of Na₂Ti₆O₁₃ whiskers (obtained from Shanghai WhiskerComposite Manufacturing Co. Ltd., China), and 8.0 g of SBR. Theparticles had an average diameter of 10 μm. The whiskers had an averagediameter of 0.8 μm and an average length of 4.5 μm. After the addition,the suspension was stirred for 50 minutes at room temperature at astirring speed of 50 rpm to form a slurry.

A 30 cm wide nonwoven PET fabric (obtained from MITSUBISHI PAPER MILLSLTD, Japan) having a thickness of about 20 μm and a weight per unit areaof about 10 g/m² was then coated with the above slurry by gravurecoating using a continuous roll coater having a doctor blade (obtainedfrom Shenzhen KEJINGSTAR Technology Ltd., China; model no.AFA-EI300-UL), in which the coating roll has an outer surface having agroove pattern. The groove pattern includes circumferentially andlongitudinally extending V-shaped grooves on the outer surface and thegrooves have equal groove widths. The longitudinally extending groovesare parallel to the longitudinal axis of the roll in order to align thewhiskers in the TD direction. The V-shaped groove has a depth of about10 μm and a width of about 7 μm. The nonwoven subsequently passedthrough an oven and dried at a temperature of 100° C. in a hot airstream. The coating speed was in the range of 1.5-1.8 meter/minute. Acoating thickness was controlled by an adjustable gap width between acoating blade and coating surface. A coated separator having a totalthickness of about 35 μm and a porosity of about 53% was obtained. Theorientation of whiskers in the protective porous layer was determinedusing an optical microscopy (obtained from Shanghai Optical InstrumentFactory, China; model no. 9XB-PC). Microscopic studies have confirmedthat over 50% of the whiskers are lying within an angle of ±30° about anaxis in the TD direction.

Example 5

Preparation of a Separator Coated with Whisker-Type and Particular-TypeMaterials

A first aqueous binder solution was prepared by dissolving 30 g of CMCin 3.2 L de-ionized water. To the first aqueous binder solution wereadded 50 g of Al₂O₃ particles (obtained from Taimei Chemicals Co. Ltd.,Japan; product no. TM-100) and 4 g of SBR. The particles had an averagediameter of 8 μm. After the addition, the suspension was stirred for 30minutes at room temperature at a stirring speed of 50 rpm to form afirst slurry.

A second aqueous binder solution was prepared by dissolving 12 g of CMCin 3.35 L de-ionized water. To the second aqueous binder solution wereadded 65 g of K₂Ti₄O₉ whiskers (obtained from Shanghai Dian YangIndustry Co. LTD, China) and 3 g of SBR. The whiskers had an averagediameter of 0.8 μm and an average length of 6 μm. After the addition,the suspension was stirred for 30 minutes at room temperature at astirring speed of 50 rpm to form a second slurry.

A 30 cm wide nonwoven PET fabric (obtained from MITSUBISHI PAPER MILLSLTD, Japan) having a thickness of about 20 μm and a weight per unit areaof about 10 g/m² was then coated with the first slurry by gravurecoating using a continuous roll coater having a doctor blade. Thenonwoven subsequently passed through an oven and dried at a temperatureof 90° C. in a hot air stream. The coating speed was in the range of1.0-1.4 meter/minute. A coating thickness was controlled by anadjustable gap width between a coating blade and coating surface, and acoated separator having a first layer of a protective porous layer witha thickness of about 8 μm was obtained. The coated separator obtainedwas further coated with the second slurry by the same continuous rollcoater in which one of the rolls was substituted by a coating rollhaving a groove pattern on its outer surface to form a second layer of aprotective porous layer. The groove pattern includes circumferentiallyand longitudinally extending V-shaped grooves on the outer surface andthe grooves have equal groove widths. The longitudinally extendinggrooves are parallel to the longitudinal axis of the roll in order toalign the whiskers in the TD direction. The V-shaped groove has a depthof about 10 μm and a width of about 7 μm. The nonwoven subsequentlypassed through an oven and dried at a temperature of 100° C. in a hotair stream. The coating speed was in the range of 1.5-1.8 meter/minute.A coating thickness was controlled by an adjustable gap width between acoating blade and coating surface. A coated separator comprising thefirst and second layers of the protective porous layer having a finaltotal thickness of about 37 μm and a porosity of about 51% was obtained.The orientation of whiskers in the protective porous layer wasdetermined using an optical microscopy (obtained from Shanghai OpticalInstrument Factory, China; model no. 9XB-PC). Microscopic studies haveconfirmed that over 50% of the whiskers are lying within an angle of±30° about an axis in the TD direction.

Example 6

Preparation of a Separator Coated with a Particular-Type Material

An aqueous binder solution was prepared by dissolving 50 g of CMC in6.55 L de-ionized water. To the aqueous binder solution were added 105 gof ZnO particles (obtained from Shanghai Xiangtian Nano Materials Co.Ltd., China; product no. XT-0806-6-2) and 7.5 g of SBR. The particleshad an average diameter of 1 μm. After the addition, the suspension wasstirred for 60 minutes at room temperature at a stirring speed of 50 rpmto form a slurry.

A 30 cm wide nonwoven PET fabric (obtained from MITSUBISHI PAPER MILLSLTD, Japan) having a thickness of about 20 μm and a weight per unit areaof about 10 g/m² was then coated with the above slurry by a continuousroll coater having a doctor blade. The nonwoven subsequently passedthrough an oven and dried at a temperature of 100° C. in a hot airstream. The coating speed was in the range of 1.5-1.7 meter/minute. Acoating thickness was controlled by an adjustable gap width between acoating blade and coating surface. A ratio of the standard deviation ofthe areal weight of the protective porous layer to the mean of the arealweight of the protective porous layer is 0.1. A coated separator havinga total thickness of about 33 μm and a porosity of about 58% wasobtained.

The tensile strength of separators of Examples 1-6 were measured. Table1 shows the tensile strength test results of separators of Examples 1-6respectively.

TABLE 1 Test Property direction Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Tensile MD 19.2 20.45 18.2 19.15 21.54 17.89strength TD 16.84 17.82 16.33 17.96 20.81 15.92 (MPa) Tensile MD/TD 1.141.15 1.11 1.07 1.04 1.12 strength ratio

The separators of Examples 1-5 exhibit a high tensile strength in boththe TD and MD directions and hence a high tearing resistance. Because ofits high mechanical strength in the TD and MD directions, the separatorcan be prevented from being damaged at the step of winding theseparator, making it possible to reduce percent defective attributed tothe separator for battery.

Based on the MD and TD data in Table 1 above, the MD and TD values ofExamples 1-2 are higher than those of Example 6. Therefore, there is anoticeable improvement in mechanical strength of separators coated witha whisker-type material over a particular-type material.

Based on the tensile strength ratio (i.e. difference in tensile strengthalong the TD direction and MD direction) in Table 1 above, the tensilestrength ratios of Examples 4 and 5 are smaller than those of Examples1-3 and 6. Therefore, there is a noticeable improvement in tensilestrength in the direction of the orientation of the aligned whisker-typematerial.

Example 7 Assembling of Pouch-Type Full Lithium-Ion BatteriesPreparation of Positive Electrodes

The positive electrode slurry was prepared by mixing 94 wt. % cathodematerial (LNMC TLM 310, obtained from Xinxiang Tianli Energy Co. Ltd.,China), 3 wt. % carbon black (SuperP; obtained from Timcal Ltd, Bodio,Switzerland) as a conductive agent, and 3 wt. % polyvinylidene fluoride(PVDF; Solef® 5130, obtained from Solvay S. A., Belgium) as a binder anddispersed in N-methyl-2-pyrrolidone (NMP; purity of ≥99%, Sigma-Aldrich,USA) to form a slurry with a solid content of 50 wt. %. The slurry wasthen uniformly spread onto aluminum foil as a current collector using adoctor blade coater (obtained from Shenzhen KejingStar Technology Ltd.,China; model no. MSK-AFA-III) and dried at 50° C. for 12 hours to obtaina cathode aluminum film.

Preparation of Negative Electrodes

The negative electrode slurry was prepared by mixing 90 wt. % of hardcarbon (HC; purity of 99.5%, obtained from Ruifute Technology Ltd.,Shenzhen, Guangdong, China) with 5 wt. % polyvinylidene fluoride (PVDF)as a binder and 5 wt. % carbon black as a conductive agent and dispersedin N-methyl-2-pyrrolidone to form another slurry with a solid content of50 wt. %. The slurry was then uniformly spread onto copper foil as acurrent collector using a doctor blade coater and dried at 50° C. for 12hours to obtain an anode copper film.

Assembling of Pouch-Type Batteries

After drying, the resulting cathode film and anode film were used toprepare the cathode sheet and anode sheet respectively by cutting intopieces of square shape in the size of 8 cm×12 cm. Two pouch-typebatteries were prepared by stacking the cathode and anode sheets in analternating manner and separated by the separator prepared in Examples 1and 2 respectively. The electrolyte was a solution of LiPF₆ (1 M) in amixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) anddimethyl carbonate (DMC) in a volume ratio of 1:1:1. The cells wereassembled in high-purity argon atmosphere with moisture and oxygencontent <1 ppm. After electrolyte filling, the pouch cells usingdifferent separators were vacuum sealed and then mechanically pressedusing a punch tooling with standard shape.

Electrochemical Measurements Nominal Capacity

The cells were tested galvanostatically at a current density of C/2 at25° C. on a multi-channel battery tester between 2.9 V and 4.2 V. Thenominal capacity for cell made with separator of Example 1 was about 20Ah, and for cell made with separator of Example 2 was about 2.6 Ah.

Cyclability Performance of Battery with Separator of Example 1

The cyclability performance of the pouch cell made with the separator ofExample 1 was tested by charging and discharging at a constant currentrate of 1 C between 3.0 V and 4.2 V. Test result of cyclabilityperformance is shown in FIG. 3. The capacity retention after 508 cycleswas about 92.7% of the initial value.

The battery using the separator of Example 1 shows good cyclability dueto uniformity of coating and high ion permeability of the separator. Inaddition, good performance of the separator of Example 1 also provesthat the separator disclosed herein has high quality after high-speedassembly in the manufacture of battery.

Cyclability Performance of Battery with Separator of Example 2

The cyclability performance of the pouch cell made with the separator ofExample 2 was tested by charging and discharging at a constant currentrate of 2.5 C between 3.0 V and 4.2 V. Test result of cyclabilityperformance is shown in FIG. 4. The capacity retention after 610 cycleswas about 95% of the initial value.

The battery using the separator of Example 2 shows good cyclability dueto uniformity of coating and high ion permeability of the separator. Inaddition, good performance of the separator of Example 2 also provesthat the separator disclosed herein has high quality after high-speedassembly in the manufacture of battery.

Nail Penetration Test

Safety tests were conducted on pouch cells made of the separators ofExamples 1 and 2 respectively, and a commercial battery cell (obtainedfrom Shandong Hengyu New Energy Co., Ltd., China) made of amulti-transition metal (Ni, Mn, Co) oxide cathode material. They had thesame capacity of 10 Ah and the size of 160 mm (L)×120 mm (W)×10 mm (T)and were in a state of charge above 95%. The nail penetration test forthe pouch cell made of the separators of Examples 1 and 2 showed nogeneration of smoke or ignition, whereas the commercial battery cellsuffered ignition immediately after penetration of nail. Therefore, ahigh level of safety with no smoke or ignition was achieved in the nailpenetration test of the cell made of the separators of the presentinvention.

While the invention has been described with respect to a limited numberof embodiments, the specific features of one embodiment should not beattributed to other embodiments of the invention. In some embodiments,the methods may include numerous steps not mentioned herein. In otherembodiments, the methods do not include, or are substantially free of,any steps not enumerated herein. Variations and modifications from thedescribed embodiments exist. The appended claims intend to cover allthose modifications and variations as falling within the scope of theinvention.

What is claimed is:
 1. A secondary-battery separator comprising, aporous base material and a protective porous layer coated on one or bothsurfaces of the porous base material, wherein the protective porouslayer comprises an organic binder and an inorganic filler, and whereinthe inorganic filler comprises a whisker-type material selected from thegroup consisting of Al₂O₃, SiO₂, TiO₂, ZrO₂, BaO_(x), ZnO, CaCO₃, TiN,AlN, MTiO₃, K₂O.nTiO₂, Na₂O.mTiO₂, and combinations thereof, wherein xis 1 or 2; M is Ba, Sr or Ca; n is 1, 2, 4, 6 or 8; and m is 3 or 6,wherein the inorganic filler further comprises a particulate-typematerial selected from the group consisting of Al₂O₃, SiO₂, TiO₂, ZrO₂,BaO_(x), ZnO, CaCO₃, TiN, AlN, and combinations thereof, wherein x is 1or 2; and wherein the protective porous layer is a two-layered structurecomprising a first layer and a second layer, wherein the first layer isadjacent to the porous base material, and the second layer is on and incontact with the first layer, and wherein the particulate-type materialresides in the first layer and the whisker-type material resides in thesecond layer.
 2. The secondary-battery separator of claim 1, wherein thewhisker-type material has a diameter from about 0.05 μm to about 3 μm,and a ratio of length to diameter from about 5 to about
 300. 3. Thesecondary-battery separator of claim 1, wherein the whisker-typematerial is unidirectionally oriented in the TD direction.
 4. Thesecondary-battery separator of claim 1, wherein the whisker-typematerial tends to align along the TD direction, and wherein at least 50%of the whisker-type material are lying within an angle of ±30° about anaxis in the TD direction.
 5. The secondary-battery separator of claim 1,wherein the particulate-type material has an average diameter from about100 nm to about 10 μm.
 6. The secondary-battery separator of claim 1,wherein the particulate-type material has an average diameter from about1 μm to about 10 μm.
 7. The secondary-battery separator of claim 1,wherein the protective porous layer has a thickness from 4 μm to about20 μm.
 8. The secondary-battery separator of claim 1, wherein the porousbase material has a thickness from about 10 μm to about 30 μm.
 9. Thesecondary-battery separator of claim 1, wherein the average thicknessratio of the second layer to the first layer is from about 1:3 to about3:1.
 10. The secondary-battery separator of claim 1, wherein the porousbase material is a non-woven fabric consisting of natural or polymericfibers, and wherein the polymeric fibers has a melting point of 200° C.or higher.
 11. The secondary-battery separator of claim 10, wherein thepolymeric fibers of the porous base material are selected from the groupconsisting of polyester, polyacetal, polyamide, polycarbonate,polyimide, polyetherether ketone, polyether sulfone, polyphenyleneoxide, polyphenylene sulfide, polyethylene naphthalene, and combinationsthereof.
 12. The secondary-battery separator of claim 11, wherein thepolyester is polyethylene terephthalate, polybutylene terephthalate or acombination thereof.
 13. The secondary-battery separator of claim 1,wherein the organic binder is selected from the group consisting of apolyester, polyamide, polyether, polyimide, polycarboxylate,polycarboxylic acid, polyvinyl compound, polyolefin, rubber, polyvinylpyrrolidone, polyacrylic acid, polyacrylate, polymethacrylic acid,polymethacrylate, polystyrene, polyvinyl alcohol, polyvinyl acetate,polyacrylamide, cellulose, cellulose acetate, cellulose acetatebutyrate, cellulose acetate propionate, carboxymethyl cellulose,cyanoethylcellulose, cyanoethylsucrose, polyurethane, nitrile butadienerubber (NBR), styrene butadiene rubber (SBR), latex,acrylonitrile-styrene-butadiene copolymer, fluorinated polymer,chlorinated polymer, and combinations thereof.
 14. The secondary-batteryseparator of claim 1, wherein the weight ratio of the inorganic fillerto the organic binder is from about 99:1 to about 1:1.
 15. Thesecondary-battery separator of claim 1, wherein the difference intensile strength of the separator along the TD direction and MDdirection is about 5% or less.
 16. The secondary-battery separator ofclaim 1, wherein the separator has a thickness from about 1 μm to about80 μm.
 17. The secondary-battery separator of claim 1, wherein theseparator has a porosity from about 40% to about 97%.
 18. Thesecondary-battery separator of claim 1, wherein the separator has anaverage pore size from about 1 μm to about 10 μm.
 19. A method forproducing a battery, comprising, inserting the separator as claimed inclaim 1 into the battery.
 20. A lithium battery comprising the separatoras claimed in claim 1.